Cyclic perketals and their use for cross-linking high density polyethylene

ABSTRACT

High density polyethylene is cross-linked with certain cyclic perketals including a group of novel cyclic perketals. Typical of the novel molecules is 3,6,6,9,9-pentamethyl-3-ethylacetate-1,2,4,5-tetraoxy cyclononane.

This is a divisional application of copending application Ser. No.772,407 filed Feb. 28, 1977, which in turn is a divisional of copendingapplication Ser. No. 688,874 filed May 21, 1976, which in turn is acontinuation-in-part of application Ser. No. 591,783 filed June 30,1975, both now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved composition and process for makingcross-linked high density polyethylene (HDPE) with certain cyclicperketals. Included within the useful cyclic perketals are a group ofnovel molecules.

2. Prior Art

Cyclic diperoxy ketals are disclosed in British Pat. Nos. 936,008 and1,329,859; U.S. Pat. Nos. 3,117,166, 3,579,541 and 3,763,275; andJapanese patent publication No. 22953/1974.

The cross-linking of polyethylene including high density polyethylenewith organic peroxides is disclosed in U.S. Pat. Nos. 3,118,866,3,436,371 and 3,846,396.

U.S. Pat. No. 3,436,371 describes the process limitations anddifficulties in employing organic peroxides for cross-linking highdensity polyethylene. For example, it is pointed out that there is asmall tolerance between the minimum temperature at which effectiveblending of the polyethylene and peroxide cross-linking agent may beaccomplished and the temperatures at which curing begins to take placeat a rate which does not afford enough time for effective blending andforming. To overcome this difficulty the patent describes the use of amodifier in combination with dicumyl peroxide cross-linking agents. U.S.Pat. No. 3,118,866 also acknowledges these same difficulties to beovercome in cross-linking high density polyethylene with organicperoxides and suggests a particular class of peroxides for accomplishingsuch a purpose.

SUMMARY OF THE INVENTION

The present invention provides a novel composition and process formaking cross-linked high density polyethylene which permits effectiveblending and forming and thereafter efficiently causes cross-linking.The composition includes high density polyethylene blended with aneffective amount or organic peroxide for the cross-linking thereof. Theorganic peroxide has the formula: ##STR1## wherein each of R₂, R₃, R₄and R₅ is selected from alkyl having from 1-4 carbon atoms; each of Rand R₁ is selected from alkyl, hydroxy alkyl and alkyl carboxylate estergroups having up to about 10 carbon atoms, ##STR2## where n=0 to 3 and Xis lower alkyl or alkoxy of up to about 5 carbon atoms; provided notmore than one of R and R₁ is methyl when all of R₂, R₃, R₄ and R₅ aremethyl. In the case of alkyl carboxylate ester groups, the carbon atomfrom the R₁ group which is attached to the methylenediperoxy carbon atomin the ring may be from either side of the carbonyl group in the ester,that is, either from the alcohol side or the carboxylic acid side of theester.

The process of this invention is carried out under conventionalconditions for cross-linking polyethylene with an organic peroxide. Thusthe high density polyethylene is mixed with the above defined organicperoxide, the peroxide constituting about 0.001-10% by weight of thehigh density polyethylene, and the mixture is subjected to sufficientheat to cause cross-linking.

Within the group of cyclic perketals defined above as suitable forcross-linking high density polyethylene, a novel group of molecules hasbeen discovered having the formula: ##STR3## wherein each of R₂ and R₃is selected from methyl and ethyl; R₁ is selected from (a)2-methyl-2-hydroxy propyl, (b) alkyl carboxylate ester groups of up toabout 10 carbon atoms, ##STR4## where n=0 to 3 and X is lower alkyl oralkoxy of up to about 5 carbon atoms.

With respect to the novel molecules, certain ones include an alkylcarboxylate ester group on the number 3 carbon atom of the heterocyclicring. Specific ester groups provided by this invention include thefollowing ones: ##STR5## In this application these ester groups shouldbe understood to be in a radical or de-protonated form for bonding tothe number 3 carbon atom of the heterocyclic ring.

DETAILED DESCRIPTION OF THE INVENTION

The novel molecules of this invention are made in accordance with thefollowing general reaction between a ketone and an alkyldihydroperoxide. ##STR6##

To make any of the novel molecules herein claimed (as well as any of theother cyclic perketals used in the cross-linking process of thisinvention) it is simply a matter of selecting the appropriate ketone anddihydroperoxide and employing it in a procedure such as the one below inwhich 3,6,6,9,9-pentamethyl 3-ethylacetate-1,2,4,5 tetraoxy cyclononaneis prepared.

EXPERIMENTAL

20.97 g of 95.4% 2,5-dimethyl-2,5-dihydroperoxy hexane (0.112 mole) wasdissolved in 65.7 g ethyl acetoacetate (0.505 mole) and 20 g isopropylacetate as a solvent at 17° C. Then 1.5 g of 35% H₂ SO₄ (0.05 mole) wasadded dropwise in 2 minutes to the stirred mixture. The temperature wasraised, and the reaction stirred for 5 hours at 30°-35° C. Then 30 ml ofbenzene was added, and the reaction mixture was transferred to aseparatory funnel. The isolated product layer was washed 3 times with100 ml 3% NaOH solution and then 2 times with 100 ml 7% NaCl solution.Finally, the product was filtered through a cake of anhydrous Na₂ SO₄ toremove all the water and concentrated under vacuum using a 35° C. waterbath. A liquid product weighing 24.15 g was recovered. 1 R analysisindicated ester carbonyl absorption at 1733 cm⁻¹ and peroxide absorptionat 858 and 881 cm⁻¹. The ketone carbonyl absorption at 1645 cm⁻¹ andhydroperoxide absorption at 3400 cm⁻¹ was not present. Product A.O.analysis: Theory, 11,02; Found, 11.00, 11.04, 99.8% pure; 73.9% yield.

The peroxides shown in Table 1 below are made by the above procedure bysubstituting the various ketones shown in the Table together withappropriate adjustment of mole ratios. When using the lower molecularweight ketone starting materials, the above experimental procedure maybe further modified by omitting the isopropyl acetate solvent.

                                      Table I                                     __________________________________________________________________________     A. Cyclic Perketals Made From 2,5-dimethyl-2,5-Dihydroperoxy Hexane           ##STR7##                       6,6,9,9-Tetramethyl-3R-3R.sub.1 -                                            1,2,4,5-Tetraoxycyclononane                                                                   10 Hr. Half-                                 Group                    Theoretical                                                                           Life Temp.,                                                                         Synthesis Result         Starting Ketone                                                                             R   R.sub.1        Molec. Wt.                                                                          Active Oxygen                                                                         °C..sup.(1)                                                                  % Yield                                                                            %                   __________________________________________________________________________                                                              Purity                Dimethyl ketone                                                                           methyl                                                                            Methyl         218.29                                                                              14.66   133   70.8 96.6                  Cyclohexane R + R.sub.1 = cyclohexyl                                                                         258.35                                                                              12.39   122.5 92.7 95.0                  Acetophenone                                                                              methyl                                                                            phenyl         280.35                                                                              11.41   139.5 40.8 95.2.sup.(2)          Methyl ethyl ketone                                                                       methyl                                                                            ethyl          232.31                                                                              13.77   129   73.5 97.4                  Methyl n-butyl ketone                                                                     methyl                                                                            butyl          260.36                                                                              12.29   130   67.3 95.1                  Diethyl ketone                                                                            ethyl                                                                             ethyl          246.34                                                                              12.99   122.3 64.4 98.8                  Ethyl isoamyl ketone                                                                      ethyl                                                                             2-methyl butyl 288.42                                                                              11.10   116.5 26.5 95.3                  4-Heptanone propyl                                                                            propyl         274.39                                                                              11.66   121.5 38   97.7                  Ethyl acetoacetate                                                                        methyl                                                                             ##STR8##      290.35                                                                              11.02   139   73.9 99.8                10.                                                                             n-Butyl levulenate                                                                        methyl                                                                             ##STR9##      332.43                                                                               9.63   136.8 87.2 83.4                  4-methyl-4-methoxy-                                                                       methyl                                                                            2-methyl-2-methoxy                                                                           290.39                                                                              11.02   127   31.9 92.9                  pentanone-2     propyl                                                        Hydroxy acetone.sup.(3)                                                                   methyl                                                                            hydroxy methyl 233.92                                                                              13.68   104   --   97.33                 Diacetone alcohol                                                                         methyl                                                                            2-methyl-      276.36                                                                              11.58   123.5 28.8 89.99                                 2-hydroxy propyl                                              3,3,5-Trimethyl                                                                           R + R.sub.1 = 3,3,5-trimethyl                                                                    300.44                                                                              10.65   122.5 70.7 89.20                 cyclohexanone                                                                             cyclohexyl                                                        Acetoxy acetone                                                                           methyl                                                                             ##STR10##     276.32                                                                              11.58   --    35.7 96.55                 2,4-Pentanedione                                                                          methyl                                                                             ##STR11##     260.32                                                                              12.29   134   44.1 92.54                 Acetoacetanilide                                                                          methyl                                                                             ##STR12##     337.40                                                                               9.48   139   78.4 98.01               __________________________________________________________________________    B. Cyclic Perketals Made From 3,6-Dimethyl-3,6-Dihydroperoxy Octane            ##STR13##                      6,9-Diethyl-6,9-Dimethyl-3R- 3R.sub.1                                        -1,2,4,5-Tetraoxy cyclononane                    Dimethyl ketone                                                                           methyl                                                                            methyl         246.35                                                                              12.99   139.5 53.2 83.60                 Methyl n-butyl ketone                                                                     methyl                                                                            butyl          288.42                                                                              11.10   124   67.7 85.16               20.                                                                             Ethyl acetoacetate                                                                        methyl                                                                             ##STR14##     318.40                                                                              10.05   126.5 Unk.sup.(4)                                                                        92.48               __________________________________________________________________________     .sup.(1) Approximately 0.1M in Benzene                                        .sup.(2) Recrystallized                                                       .sup.(3) Example 1, U.S. Pat. No. 3,579,541                                   .sup.(4) Losses during isolation                                         

In accordance with the present invention the cyclic perketals includingthe novel molecules of this invention are mixed or blended with highdensity polyethylene. High density polyethylene is defined in the priorart as having a density of about 0.94-0.96. (Modern PlasticsEncyclopedia 1974-1975, page 82.) When the cyclic perketal is blendedwith such material cross-linking is accomplished by subjecting themixture to a sufficient temperature, usually within a preferred range ofabout 150°-200° C. The amount of organic peroxide utilized is consistentwith the prior art and will usually be in the range of about 0.001-10%by weight of the polyethylene.

In the preferred embodiment heating for purposes of cross-linking isapplied during the procedure known as rotational molding. Rotationalmolding is a process intended primarily for the manufacture of hollowobjects. In this process the solid or liquid polymer is placed in amold; the mold is first heated and then cooled while being rotated abouttwo perpendicular axes simultaneously. During the first portion of theheating stage when molding powdered material, a porous skin is formed onthe mold surface. This gradually melts as the cycle progresses to form ahomogenous layer of uniform thickness. However, when molding a liquidmaterial, it tends to flow and coat the mold surface until the geltemperature of the resin is reached, at which time all flow ceases. Themold is then indexed into a cooling station, where forced air, waterspray or a combination of both cool the mold. It is then positioned in awork zone, where the mold is opened, the finished part removed and themold recharged for the following cycle. For more details as to theprocess and apparatus used in rotational molding see Plastic Design AndProcessing, February 1974, pages 19-23 and March 1974, pages 21-24.

In the preferred embodiment for rotational molding of high densitypolyethylene it is preferred to use cyclic perketals of the followingformula: ##STR15## wherein each of R₂, R₃, R₄ and R₅ is selected fromalkyl having from 1-4 carbon atoms; each of R and R₁ is selected fromalkyl, hydroxy alkyl and alkyl carboxylate ester groups having up toabout 10 carbon atoms, ##STR16## where n=0 to 3 and X is lower alkyl oralkoxy of up to about 5 carbon atoms; provided not more than one of Rand R₁ is methyl when all of R₂, R₃, R₄ and R₅ are methyl, and where Rand R₁ are alkyl, one of R and R₁ is methyl. These above perketals aredesirable because they exhibit a satisfactory pot-life for rotationalmolding.

During the initial processing of high density polyethylene the organicperoxide is mixed and blended thoroughly with the polyethylene. To besatisfactory the organic peroxide should not cause excessive prematurecross-linking during this phase of the processing. The cross-linking isdesired only upon actual molding of the blended polyethylene. Thefollowing experimental work is designed to show the satisfactorypot-life obtainable with the cyclic perketals of this invention. In thiswork the equipment used simulates the milling or commonly used Banburymixing step wherein all the ingredients such as anti-oxidants, fillersand the like which are normally added are mixed into moltenpolyethylene. Following this mixing step the blended material isconventionally sheeted out and pelletized or ground into a powder forlater use in a mold such as a rotational molder or an extruder. Morespecifically, the data in Table II below was obtained utilizing thefollowing procedure.

A Brabender Plasticorder with a Roller-5 type mixing head at a rotorspeed of 30 RPM was used for these tests. Test conditions were:

A mixing head temperature of 160° C. was used. The resin used wasSode'fine Manolene ER63ONS HDPE (density range equals 0.946-0.968 g/cc;melt index equals 30). For these tests, 40.00 g of powdered resin wasadded directly to the mixing head. After 15 minutes, the desired amountof peroxide diluted in n-hexane was added by a syringe to the mixinghead. The maximum torque reached in 65 minutes (50 minutes net) wasrecorded, and the run terminated. Net torque is equal to the maximumtorque reached in 65 minutes minus the torque of virgin resin at 65minutes. Each peroxide was tested twice.

                                      Table II                                    __________________________________________________________________________    Pot-Life of Various Cyclic Perketals in HDPE Using the Brabender              Plasticorder at 160° C. Compared to Commercial Peroxides               Each Peroxide used at a Molar Active Oxygen Equivalent to                     1 phr 2,5-Dimethyl-2,5-Di-t-butyl peroxy Hexyne-3                             __________________________________________________________________________      Peroxide    phr Used.sup.1    Net Torque, Mg.sup.2                          __________________________________________________________________________      2,5-Dimethyl-2,5-di-t-                                                                    1                 570                                             butyl peroxy hexyne-3                                                         α,α'-Bis(t-butyl peroxy)                                                      1.17              4685.sup.3                                      diisopropyl benzene                                                         __________________________________________________________________________    A. Cyclic Perketals Made From 2,5-Dimethyl-2,5-Dihydroperoxy Hexane                         Group                  Net Torque,                                Starting Ketone                                                                           R   R.sub.1       phr Used                                                                           Mg.                                      __________________________________________________________________________      Hydroxy acetone                                                                           methyl                                                                            hydroxy methyl                                                                              0.82 3858.sup.4                                 Acetone     methyl                                                                            methyl        0.76 362                                        Methyl ethyl ketone                                                                       methyl                                                                            ethyl         0.81 625                                        Methyl n-butyl ketone                                                                     methyl                                                                            butyl         0.91 477                                        Diacetone alcohol                                                                         methyl                                                                            2-methyl-     0.965                                                                              732                                                        2-hydroxy propyl                                             8.                                                                              Ethyl acetoacetate                                                                        methyl                                                                            ##STR17##     1.00                                                                               242                                      9.                                                                              n-Butyl levulenate                                                                        methyl                                                                            ##STR18##     1.16                                                                               217                                     10.                                                                             Diethyl ketone                                                                            ethyl                                                                             ethyl         0.86 1317                                       Ethyl isoamyl ketone                                                                      ethyl                                                                             2-methyl butyl                                                                              1.01 1272                                       4-Heptanone propyl                                                                            propyl        0.96 1270                                       Cyclohexanone                                                                             R + R.sub.1 = cyclohexyl                                                                        0.90 270                                      __________________________________________________________________________    B. Cyclic Perketals Made From 3,6-Dimethyl-3,6-Dihydroperoxy Octane             Acetone     methyl                                                                            methyl        0.86 162                                      __________________________________________________________________________     .sup.1 On a 100% purity basis                                                 .sup.2 50 minutes after peroxide added compared to virgin resin               .sup.3 Cross-linked in 25.3 min.                                              .sup. 4 Cross-linked in 19.4 min.                                        

In the above Table II the first two peroxides are included forcomparison purposes. Peroxide 1 is a commercially available materialwith a suggested use similar to that of this invention. Peroxide 2 istypical of prior art peroxides for cross-linking polyethylene such asthose disclosed in U.S. Pat. No. 3,118,866.

As may be seen from the results in Table II, the cyclic perketal madefrom hydroxyacetone (Example 1, U.S. Pat. No. 3,579,541) is unsuitablebecause it cross-links the polyethylene at 160° C. It is also noted thatperoxides 10, 11 and 12 have a substantially higher net torque valueover the commercially available peroxide 1. In view of these propertiesperoxides 10, 11 and 12 may be less desirable for cross-linking highdensity polyethylene in a rotational molding process as compared withthe other cyclic perketals of the present process. However, evenperoxides 10, 11 and 12 demonstrate suitable properties forcross-linking high density polyethylene during molding procedures.

Table II indicates those types of cyclic perketals which demonstratesatisfactory pot-life properties so as to be useful for cross-linkinghigh density polyethylene. While a cyclic perketal may have a suitablepot-life, in order to function in the process of this invention it mustadditionally effect the desired cross-linking reaction. The followingexperimental work illustrates the types of cyclic perketals which areeffective cross-linking agents for high density polyethylene.

Tables III and IV below illustrate the effectiveness of the molecules ofthis invention. The data in Table III was generated by causingcross-linking to occur by press molding in accordance with the followingprocedure. Effectiveness for cross-linking is reported in terms ofpercent of cross-linking by the conventionally used percent gel test fordetermining the degree of cross-linking in cross-linked polyethylene.

PRESS MOLDING OF HDPE (TABLE III)

The desired amount of peroxide was dry-blended into 30.00 g of powderedresin by stirring for 5 minutes with a spatula. The resin used wasSode'fine Manolene ER63ONS HDPE (density range equals 0.946-0.968 g/cc;melt index equals 30). Solid peroxides were dissolved in n-hexane andadded to the resin in a round bottom flask. After mixing, the solventwas removed under reduced pressure using a rotating evaporator and a 40°C. water bath. Platen temperatures on the press were checked with asurface pyrometer and were 390°±5° F. Press time was varied from 15-30minutes depending upon the peroxide used. Cure time started when the ramgauge read 1,000 PSIG. Final ram pressure was between 4,000-8,000 PSIG.It took 6 minutes to cool the press to 245°-250° F. (platenthermometers) and release the pressure. Trays were removed from themolds within two minutes and cooled quickly in a water bath.

To determine the %wt gel, approximately 0.3000 g sample was cut into 6-7pieces and placed inside a stainless steel screen pouch. These poucheswere extracted in 2 liters of boiling xylene containing 10 g ofPlastanox 2246 anti-oxidant for 16 hours and then dried in an oven at170° C. for 4 hours.

The %wt gel was calculated by the following formula: ##EQU1## wherein:W₁ =wt. of sample, g

W₂ =wt. of sample+screen, g

W₃ =wt. of sample+screen after extraction, g

0.0036=blank value for resin without peroxide.

                                      Table III                                   __________________________________________________________________________    Cyclic Perketals Made From 2,5-Dimethyl-2,5-Dihydroperoxy Hexane              and Tested as Cross-Linking Agents of HDPE                                     ##STR19##                        6,6,9,9-Tetramethyl-3R-  3R.sub.1                                           -1,2,4,5-Tetraoxycyclononane                                                                 % Wt Gel when                                  Group             Amt. Used    Press Molded at                                                               390° C.                 Starting Ketone R   R.sub.1       phr.sup.1                                                                          Moles (×10.sup.-3)                                                              Molded 15                                                                             Molded 30              __________________________________________________________________________                                                           Min.                     Dimethyl ketone                                                                             methyl                                                                            methyl        (a) 0.76                                                                           3.48     1.8     --                                                      (b) 1.00                                                                           4.58    48.7     --                      Cyclohexanone R + R.sub.1 = cyclohexyl                                                                        (a) 0.90                                                                           3.48     0       --                                                      (b) 1.50                                                                           5.81     0       --                      Acetophenone  methyl                                                                            phenyl        (a) 0.50                                                                           1.78     0        --                                                     (b) 0.75                                                                           2.68     0       --                                                      (c) 1.00                                                                           3.57    16.2     --                                                      (d) 1.50                                                                           5.35    46.6     --                      Methyl n-butyl ketone                                                                       methyl                                                                            butyl         (a) 0.75                                                                           2.88    93.7     --                                                      (b) 0.91                                                                           3.50    94.7     --                                                      (c) 1.50                                                                           5.76    95.8     --                      Diethyl ketone                                                                              ethyl                                                                             ethyl         (a) 0.75                                                                           3.04    93.5     --                                                      (b) 0.86                                                                           3.49    95.5     --                                                      (c) 1.50                                                                           6.09    97.1     --                      Ethyl acetoacetate                                                                          methyl                                                                             ##STR20##    (a) 0.50 (b) 0.75 (c) 1.00  (d) 1.25                                          (e) 1.50                                                                           1.72 2.58 3.44 4.31 5.17                                                              12.5 73.9 91.3 92.2                                                                    -- 86.4 84.0 90.5                                                            2.2                      n-Butyl levulenate                                                                          methyl                                                                             ##STR21##    (a) 0.75 (b) 1.16 (c)                                                               2.26 3.49 4.51                                                                       23.3 48.8 84.3                                                                        28.4  -- 87.3            2,5-dimethyl-2,5-                                                                            --   --          (a) 0.50                                                                           1.75    96.3     --                      Di(t-butylperoxy)Hexyne-3.sup.2)                                                                              (b) 0.75                                                                           2.62    98.5.sup.3                                                       (c) 1.00                                                                           3.49    99.0     --                      Blank, Resin only                                                                            --   --          (a) None                                                                            --      0.4     --                    __________________________________________________________________________     .sup.1 On a 100% purity basis                                                 .sup.2 A commercial dialkyl peroxide normally used to crosslink HDPE          .sup.3 Molded 10 min.                                                    

The data in Table IV below is also offered to show effectiveness incross-linking. In this instance results are reported in terms of nettorque as measured in a Torque Rheometer. As with the data in Table II,a Brabender Plasticorder with a Roller-5 type mixing head at a rotorspeed of 30 RPM was used for these tests.

A mixing head temperature of 180° C. was used. The resin used wasPhillips Marlex BMN 5565 HDPE (density equals 0.955 g/cc; melt indexequals 6.5). The same weight of resin was used, and same procedurefollowed as with Table II, except the diluted peroxide was added at 16minutes. If the maximum torque was not reached in 76 minutes (60 minutesnet), the run was terminated. Net torque is equal to the maximum torqueminus the torque at 16 minutes. Time to reach maximum torque is the timeat maximum torque minus 16 minutes.

                                      Table IV                                    __________________________________________________________________________    A. Cyclic Perketals Made From 2,5-Dimethyl-2,5-Dihydroperoxy Hexane           and Tested as Cross-Linking Agents of HDPE Using the Brabender                Plasticorder at 180° C. & 200° C.                                ##STR22##                       6,6,9,9-Tetramethyl-3R- 3R.sub.1                                             -1,2,4,5-Tetraoxycyclononane                                                 Amt. Used  Net Torque, Mg.sup.2                                                                   Time to Reach                           Group                  Moles Head Temp., °C.                                                                 Maximum Torque, Min.         Starting Ketone                                                                          R   R.sub.1       phr.sup.1                                                                          (×10 .sup.-3)                                                                   180                                                                               200                                                                              180 200                    __________________________________________________________________________      Dimethyl ketone                                                                          methyl                                                                            methyl        (a) 0.5                                                                            2.29    665                                                                              2010,                                                                             60.0.sup.3                                                                        47.0, 48.3                                            (b) 1.0                                                                            4.58    790                                                                               -- 60.0.sup.3                                                                         --                      Cyclohexanone                                                                            R + R.sub.1 = cyclohexyl                                                                        (a) 0.5                                                                            1.94    615                                                                               --  60.0.sup.3                                                                        --                                                   (b) 1.0                                                                            3.87    1000                                                                              -- 60.0.sup.3                                                                         --                      Acetophenone                                                                             methyl                                                                            phenyl          0.5                                                                              1.78    570                                                                               -- 60.0.sup.3                                                                         --                      Methyl ethyl ketone                                                                      methyl                                                                            ethyl           0.5                                                                              2.15  (a) 2010                                                                            -- 45.3                                                                               13                                                              (b) 2065                                                                            -- 44.0                                                                               --                      Methyl n-butyl ketone                                                                    methyl                                                                            butyl           0.5                                                                              1.92    2440                                                                             4000                                                                              39.0                                                                              17.5                     Diethyl ketone                                                                           ethyl                                                                             ethyl           0.5                                                                              2.03    2735                                                                              -- 35.0                                                                               --                      Ethyl isoamyl ketone                                                                     ethyl                                                                             2-methyl butyl                                                                                0.5                                                                              1.73    1990                                                                              -- 12.0                                                                               --                      4-Heptanone                                                                              propyl                                                                            propyl          0.5                                                                              1.82    3250                                                                              -- 27.8                                                                               --                      Ethyl acetoacetate                                                                       methyl                                                                             ##STR23##    (a) 0.5 (b) 1.0                                                                    1.72 3.44                                                                             1980   3200                                                                      3050  --                                                                          56.4 36.6                                                                         28.3   --              10.                                                                             n-Butyl levulenate                                                                       methyl                                                                             ##STR24##    (a) 0.5 (b) 1.0                                                                    1.50 3.01                                                                             1240   1855                                                                       -- 60.0.sup.3 54.0                                                                    --  --                  2,5-Dimethyl-2,5-Di(t-                                                                     --                                                                                --            0.5                                                                              1.75    1935                                                                              -- 26.0                                                                               --                      butyl peroxy)Hexyne-3.sup.4                                                   4-Methyl-4-methoxy                                                                       methyl                                                                            2-methyl-2-   (a) 0.5                                                                            1.72    365                                                                               -- 40.0.sup.5                                                                         --                      pentanone-2    methoxy propyl                                                                              (b) 0.85                                                                           2.93    305                                                                               -- 40.0.sup.5                                                                         --                                                   (c) 0.85                                                                           2.93    370                                                                               -- 40.0.sup.5                                                                         --                      Diacetone alcohol                                                                        methyl                                                                            2-methyl-2-hydroxy                                                                          (a) 0.5                                                                            1.18    1470                                                                              -- 17.2                                                                               --                                     propyl        (b) 1.0                                                                            3.62    4400                                                                              -- 17.5                                                                               --                      3,3,5-Trimethylcyclo-                                                                    R + R.sub.1 = 3,3,5-trimethyl                                                                     0.5                                                                              1.66    440                                                                                --                                                                              60.0.sup.3                                                                         --                      hexanone   cyclohexyl                                                         Acetoxy acetone                                                                          methyl                                                                             ##STR25##      0.5                                                                              1.81    2700                                                                              -- 39.7                                                                               --                      2,4-Pentanedione                                                                         methyl                                                                             ##STR26##      0.5                                                                              1.92    1880                                                                             3540                                                                              49.5                                                                              20.8                     Acetoacetanilide                                                                         methyl                                                                             ##STR27##    (a) 0.5 (b) 0.77                                                                   1.48 2.29                                                                             -- 1770, 1655 2330                                                                    -- 18.5, 19.0             __________________________________________________________________________                                                           16.5                   B. Cyclic Perketals Made From 3,6-Dimethyl-3,6-Dihydroperoxy Octane                                           6,9-Diethyl-6,9-Dimethyl-3R-3R.sub.1                                          -1,2,4,5-                                                                     Tetraoxy cyclononane                            Dimethyl ketone                                                                          methyl                                                                            methyl        (a) 0.5                                                                            2.03    1795                                                                              -- 60.0.sup.3                                                                         --                                                   (b) 1.0                                                                            4.06    3315                                                                               --                                                                              35.7                                                                               --                                                   (c) 1.0                                                                            4.06    3445                                                                              -- 36.0                                                                               --                      Methyl n-butyl ketone                                                                    methyl                                                                            butyl           0.5                                                                              1.73    1850                                                                              -- 50.0                                                                               --                    20.                                                                             Ethyl acetoacetate                                                                       methyl                                                                             ##STR28##    (a) 0.5 (b) 1.0                                                                    1.57 3.14                                                                             1120   2615                                                                       -- 60.0.sup.3 45.8                                                                    --  --                __________________________________________________________________________     .sup.1 Parts per 100 parts resin, on a 100% purity basis                      .sup.2 Meter-grams                                                            .sup.3 Run terminated 60 min. after peroxide added                            .sup.4 A commercial dialkyl peroxide normally used to crosslink HDPE          .sup.5 Run terminated 40 min. after peroxide added                       

Of the cyclic perketals made from 2,5-dimethyl-2,5-dihydroperoxy hexanewhich survive the pot-life test, the following ones fall to cross-linkHDPE as may be seen in Tables III and IV:

    ______________________________________                                                       Group                                                          Starting Ketone  R        R.sub.1                                             ______________________________________                                        1. Acetone       methyl   methyl                                              2. Cyclohexanone R + R.sub.1 = cyclohexyl                                     3. 3,3,5-Trimethyl                                                                             R + R = 3,3,5-trimethyl                                       Cyclo hexanone  cyclohexyl                                                   4. Acetophenone  methyl   phenyl                                              5. 4-methyl-4-methoxy                                                                          methyl   2-methyl-                                            pentanone-2              2-methoxy propyl                                    ______________________________________                                    

Surprisingly, the cyclic perketal made with diacetone alcohol(4-hydroxy-4-methyl pentanone-2) works well. All three cyclic perketalsmade from 3,6-dimethyl-3,6-dihydroperoxy octane work well, including theone made from acetone.

We claim:
 1. An organic peroxide of the formula: ##STR29## wherein eachof R₂ and R₃ is selected from methyl and ethyl.
 2. The organic peroxideof claim 1 wherein R₂ and R₃ equal methyl.
 3. The organic peroxide ofclaim 1 wherein R₂ and R₃ equal ethyl.